When converting electric power from one form to another, it is frequently necessary to switch back and forth to produce an AC current of a particular frequency. Sometimes this is an intermediate step to create a DC current at a desired voltage level, in other situations an AC current is the desired output.
When periodic electrical switching is performed at a frequency on the order of 60 Hz and with a current in the range of 5 Amps to 50 Amps, it is necessary to employ capacitors in the range of 200 to 1,000 micro Farads to absorb and smooth the ripple current produced by the circuit inductance. A natural choice for this application is the multi-layer polymer (MLP) capacitor. An MLP capacitor with capacitance of 300 micro Farads can be built having modest physical dimensions (about 300 cm3) and having a low (≦4Ω) equivalent series resistance (ESR). A low ESR is important to prevent the capacitor from heating up too much during high-current operation.
Unfortunately, as large enough currents are routed through a MLP capacitor, the capacitor does heat up, despite the low ESR. This causes the water present in the MLP capacitor to expand, eventually causing the metal housing of the capacitor to buckle, if the temperature gets too high (>81° C.). One technique for addressing this problem is to clamp the MLP capacitor. This, however, requires that pressure be applied evenly over the exterior wall having the greatest extent, requiring some careful adjustment of the clamping mechanism, sometimes involving the use of springs to measure the pressure exerted on various parts of the exterior wall. Another practice is to heat sink or otherwise cool the MLP capacitor.